Small Scale Foundries for Developing Published by: Intermediate ...

make the process more economical in some
circumstances.
Cenir@igaC Casting
If molten metal is poured into a rapidly spinning
tubular metal permanent mould a
cylindri::$ casting can be made.
On a large scale, this process is used for
cast iron and SC (spheroidal graphite)
iron water-pipes; smaller centrifugally cast
bronze tubes are used for making bearings,
and cast iron cylinder liners for engines are
also produced by this process.
The speed of rotation has to be related to
the casiiligs being made. For generai casrings,
such as bronze bushes, about 200 rpm
is norma!, whilst for the highest auaiiry cast
iron cylinder liners speeds up to 4i)O rpm are
used.
A centrifugal casting machine consists of
a cast iron tubular mould, a motor and driving
system for the spif,ning action, removable
end plates and - usually - a water
spray system to cool the outside of the
mould. Machines may be purchased in a
range of sizes; simple centrifugal casting
machines can be made in a well equipped
engineering workshop.
This process should not be confused with
continuous casting in which metai fiows
slowly out of a furnace through a watercooled
collar, emerging as a rod or bar of
solid metal. Continuous casting is not easy
to control and the large amounts of material
produced mean that it is a process unlikely
to be of interest to smaii scaie foundry
enterprises.
Clay-Bonded Sand Moulds
Most castings are made in sand moulds. Not
every sand is suitable for the production of
castings. In order to produce a mould, the
sand grains must be stuck or bonded
together. The bond is usually provided by
clay. IMar sand deposits contain sufficient
natural. c!ay for this bond. Such “natural”
sands with about 12% to 15% of clay are
found in many-parts of the world. It is possible
by laboratory testing to determine
whether a sand bill be suitable for a foundry
process, although it is usually better to carry
out actual trials in a foundry.
Many foundries use sand which contains
no natural Ciay, and add clay separateiy
(usuaily between 5% and iGVo by weightj.
This type of sand must be clean, and in particular
free from mica, volcanic ash, crushed
sea shell or coral. The grain size should be
fairly uniform, the fineness determining the
smoothness of the casting. The grain shape
should be round, or sub-angular; wind
blown sands or sands which have been
washed as a by-product of mineral extraction
are often suitable. Lake sands and river
sands are frequently used, but sea beach
sand is sometimes contaminated with shell
or salt. Sand from some deposits may
require washing and cleaning before use, or
at very least sieving to remove lumps or
foreign matter. Washed sand is likely to contain
too much water for foundry use and
may require drying - a simple sand dryer is
not difficult to construct.
C!ay and Moisture
Where separate clay additions are made, the
best type of clay to use is bentonite. This
material is available commercially for
foundry, and also for oil-well use. Fireclay
and other types of clay are also used in
certain cases.
Sand bonded with clay must have the
right amount of moisture in order to make
good moulds. The water content depends
upon the amount and type of clay present,
varying from about 3% to about 7% by
weight. Tile water content can be measured
by accurately weighing a sample of sand
before and after drying in an oven - other
chemical and electrical methods can ais0 be
used in laboratories.
Mould Drying
Moulds made in clay-bonded sand, with a
natural or synthetic clay, are suitable for the
production of relatively thin sectioned iron,
steel and non-ferrous castings without drying.
Moulds for heavy castings produced in